1. Field of the Invention
The present invention relates to oscillators, and it particularly relates to an oscillator that oscillates signals of a predetermined frequency.
2. Description of the Related Art
A voltage controlled oscillator is used in optical pickups or phase-locked loops (PLL), for instance. Generally, an oscillation frequency is adjusted according to a control voltage applied and then a signal of the thus adjusted oscillation frequency is outputted. In an example of a conventional voltage controlled oscillator, an inverting amplifier, a first charge/discharge circuit and a second charge/discharge circuit are electrically coupled to form a circuit. In this structure, the phase of an inverted voltage signal from the inverting amplifier is delayed in stages at the first and the second charge/discharge circuit and the output of the second charge/discharge circuit is again inputted to the inverting amplifier. After a full circle, the phase of an inverted voltage signal returns to the same phase as the original one, so that the voltage controlled oscillator can keep oscillating by repeating the above processing. The oscillation frequency of the voltage controlled oscillator is determined mainly in response to the magnitude of the charge/discharge current at the first and the second charge/discharge circuit, and the magnitude of the charge/discharge current is controlled by an easily controllable control current which has a current value level higher than the charge/discharge current (See, for instance, Reference (1) in the following Related Art List).
Related Art List
(1) Japanese Patent Application Laid-Open No. Hei06-37599.
According to such conventional technology, the charge/discharge current, even when it is very small, is controlled by control current, so that stable oscillation output can be achieved even at low oscillation frequencies by stabilizing the level of current values for the control. For the oscillation at high oscillation frequencies, on the other hand, there are generally the following problems to be solved. When an oscillation signal at a high oscillation frequency is to be generated and further to be converted from into a current signal by a field effect transistor (FET) (hereinafter this FET will be referred to as “converting FET”), the distortion of an oscillation signal due to the conversion is in general likely to occur. When this distortion results in high-order harmonic contents, the EMI (electromagnetic interference) characteristics tend to deteriorate. Also, when the oscillation frequency of the oscillation signals outputted finally from the oscillator is made high and the amplitude of said oscillation signals is enlarged, the power consumption therefor will generally increase. If the oscillator is incorporated into a battery-driven device or the like, lower power consumption is desired but the conversion efficiency from the voltage signal to the current signal needs to be improved.
On the other hand, the LSI vendors who supply oscillators built in LSIs (large-scale integrated circuits) or the like find it desirable if such LSIs can be used for general purpose to gain mass-production effect. And set makers, who incorporate the LSIs into certain devices or the like, desire that an oscillator sets variably the amplitude of output signals in accordance with a condition required by a device and it operates at low power consumption. For that purpose, the oscillator is required to have appropriate characteristics relative to the amplitude of signals to be outputted, the power consumption and the like. Moreover, if the set maker applies the oscillator to inside predetermined equipment and the amplitude of output signals is set to a large value, certain requirements need to be met as to the distortion of a waveform or the EMI characteristics.